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Keeping boxwood blight at bay is best option for now

March 14, 2018

By

Elizabeth O’Connell

When I can’t garden I take horticulture classes, and this year plant diseases were a focus of several sessions, especially a fungus called boxwood blight.

It was first detected in plants in Virginia and Connecticut in 2011. By 2016 it had spread to plants in a dozen states including Illinois. Wisconsin is now trying to prevent it from gaining a foothold in the state.

Boxwood blight (Cylindrocladium buxicola) is a fungal disease that causes black stem cankers and leaf drop. What first appears as small black spots on leaves rapidly leads to complete defoliation and the death of the plant. The blight is most likely to appear when the weather is cool and wet, conditions we frequently see in Port. It’s highly contagious and will also infect pachysandra.

Several of the boxwoods most resistant to blight — ‘Green Mound,’ ‘Glencoe,’ and ‘Chicagoland Green’— are also recommended for our area because of their hardiness. Boxwoods have shallow roots, so underplanting near them isn’t ideal. To avoid creating conditions for blight to thrive, it’s also suggested to lay drip irrigation under boxwoods to keep the leaves as dry as possible in case they need supplemental watering.

The number one evergreen alternative to boxwood is Ilex glaba, or inkberry. This is a member of the holly family native to the eastern and southern parts of the United States. This native is 6 to 8 feet tall and the shrubs become leggy and open as they mature. They also have a tendency to sucker, so they are high maintenance in home landscapes.

Several cultivars have been developed in Connecticut to overcome these issues, and they’re finding widespread landscape use as boxwood blight spreads. ‘Shamrock’ and ‘Green Magic’ are the most recommended of the hybrid inkberry boxwood replacements. Both are low growing, 3 to 5 feet tall, and more compact and leafier than their wild relatives. They retain the natives’ tolerance of salt and wet ground but still need acidic soils to thrive.

And that’s a problem when suggesting inkberry in Wisconsin as a replacement for boxwood. Much of the state, including the lakeside counties, are sitting on limestone, which creates soil as far from acidic as possible. One constant problem for gardeners in this area is dealing with high pH (alkaline) soil conditions. Even the water we use is alkaline. Many acid-loving plants here develop nutrient deficiencies, including chlorosis, and even if they don’t die they seldom thrive without continuous soil amendment.

For local gardeners who want a low evergreen hedge, the other top recommendations for boxwood replacements are just as problematic. Low-growing spruces are popular, but spruces here are also developing fungal diseases.

Right now the best bet for boxwood lovers is to keep boxwood blight at bay. The University of Wisconsin Plant Disease Clinic recommends purchasing boxwoods grown in-state, and that goes for pachysandra, too, even if Illinois nurseries have bargain prices. If problems develop on established boxwood plants, send samples to the state lab for blight testing. Remove and burn any diseased boxwoods immediately. These kind of measures haven’t stopped other pests and diseases, but right now it’s the only way to preserve this beautiful favorite.